A. Field of the Invention
The present invention relates to methods and apparatus for propelling flexible borescopes. More particularly, the invention relates to devices for propelling borescope heads by reaction forces generated in response to fluid expelled from jet thrusters.
B. Discussion of Background Art
In a co-pending U.S. Pat. application, Ser. No. 854,295, filed Apr. 21, 1986, and assigned to the same assignee as the present application, a novel and highly effective method and apparatus for propelling borescopes was disclosed. The apparatus disclosed in that co-pending application uses one or more reaction jets attached to the head of a flexible borescope to propel and guide the borescope head to inaccessible locations which it is desired to view. Each jet is powered by compressed air or other pressurized fluid, supplied to the jet by a flexible tube running back alongside or through the borescope cable to an external source of pressurized fluid. The basic embodiment of the invention disclosed in the above-identified application uses at least one rearward directed reaction jet to propel in a forward direction the borescope head to which the jet is attached.
Our continued testing of various embodiments of fluid propelled borescopes employing the teachings of the above-identified co-pending application have proven that the novel methods and apparatus disclosed therein are highly effective. In addition to the vast improvement in the speed and accuracy with which flexible borescope tips could be guided through complex machines such as jet turbine engines, we have discovered further advantageous features of our fluid propelled borescopes. For example, we found that under certain conditions, the rearward directed stream of propelling fluid flowing along the borescope tip and cable tends to adhere to the surfaces of the tip and cable, because of a boundary layer effect. Moreover, this boundary layer of fluid caused the tip and cable to tend to adhere to adjacent walls of passages through which the cable and tip were guided, because of the Bernoulli effect. In fact, the fluid flow could be adjusted so that the borescope head and a portion of the cable trailing behind the head could even be adhered to the ceiling of a structure, the Bernoulli force counteracting the force of gravity.
The jet tubes or thrusters disclosed in the above-identified co-pending application and tested by us have the general shape of a small tube bent into the shape of a hairpin. One end of the tube is connected to a flexible hose providing pressurized fluid to the thruster. The other end of the tube is directed generally rearward, or in a desired thrust direction, and typically has a tapered, narrowing orifice to increase the velocity of expelled fluid.
Thrusters having the general shape of a small diameter tube bent into a doubled-back, hairpin curve or U-tube shape as described above, were found to be highly effective for their intended purpose. However, an inherent limitation of the U-tube thruster is its relatively large cross-sectional profile. Since borescopes are often used in applications requiring them to be threaded through narrow and labyrinthine passageways, reduction of the cross-sectional profile of thrusters used with fluid propelled borescopes would be highly desirable. With that and other considerations as a motivating factor, the present invention was conceived of.